The nicotinic acetylcholine receptors are a major class of excitatory receptor in the brain. As such, they are important in normal physiology and in neuropathological states. Study of these receptors is important for research and diagnosis of neurodegenerative diseases. For example, the nicotinic acetylcholine receptor has been implicated in various neuropathological and physiological states, including Alzheimer's disease and addiction to tobacco products. With respect to tobacco addiction, these receptors are the sites at which the addictive drug nicotine produces its effects. However, in patients who have died from Alzheimer's disease, the densities of nicotinic acetylcholine receptors in the brain are reduced. Currently, Alzheimer's disease is definitively diagnosed only at autopsy, and rates of false positive diagnoses in living patients range from 19% to 45%. Science, 266, Dec. 16, 1994. A noninvasive method of diagnosing neurodegenerative diseases in the living human brain is highly desirable.
To date, an appropriate agent for labeling and quantifying the nicotinic acetylcholine receptors in living human brain is not available. These receptors have been characterized heretofore by radioligand binding and electrophysiological assays. Sargent, Annu. Rev. Neurosci., 16:403 (1993). However, in vivo imaging studies of nicotinic acetylcholine receptors have been impeded by lack of a suitable radioligand.
The prior art discloses attempts to image neuroreceptor populations in living animals using various labeled compounds. However, until now, none of the labeled compounds have been successful in imaging and quantifying nicotinic acetylcholine receptors in the living brain.
For example, radiolabeled nicotine has been prepared and evaluated as a ligand for localizing and quantifying nicotinic cholinergic receptors ("nAChRs") by Positron Emission Tomography ("PET"), but its rapid clearance and relatively low affinity have limited its usefulness. Recent studies in mice have used labeled cytisine, another ligand with high affinity for nAChRs. Those studies indicate that this ligand does not clear as rapidly from the brain as nicotine and does not have as much non-specific binding. Both radiolabled nicotine and cytisine present limitations as in vivo tracers due to their non-specific binding in the brain and rapid clearance from the brain.
Recently, epibatidine has been discovered and various pharmacological uses for it have been suggested. Epibatidine (exo-2-(2-chloro-5-pyridyl)-7-azabicyclo[2.2.1]heptane), an extract of frog skin, has recently been discovered to produce antinociception, indicating central activity after peripheral administration, and extremely high potency for central nAChRs in vitro. Badio, Mol. Pharmacol., 45:563 (1994). Epibatidine is disclosed U.S. Pat. Nos. 5,314,899 and 5,462,956. However, these patents do not disclose fluorine as a specific substituent. PCT application WO94/22868 to Shen et al. discloses the epibatidine analogue (.+-.)-exo-2-(2-fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ("FPH") for its analgesic activity. Methods for preparing epibatidine may found in JP 702087 and JP 7033771.
Previously, epibatidine has been studied for use as an analgesic and nicotinic agonist. It has been learned that enantiomers of epibatidine have similar inhibition of nicotine binding to nicotinic sites in rat brains. Mol. Pharmacol., 45:563-69 (1994).
Other labeled compounds, other than FPH, have been studied for use as ligands for the neuronal nicotinic receptor, for example, [.sup.11 C]N-methylepibatidine and [.sup.18 F]fluoroethylepibatidine, Journal of Labelled Compounds and Radiopharmaceuticals, 37:355-56 (1995), and nicotinic binding assays on the brains of patients who died of dementia of the Alzheimer type have been performed using [.sup.3 H]acetylcholine and [.sup.3 H]nicotine as radioligands. Neurochemical Research, 14:745-50 (1989).
Attempts have been made to visualize nicotinic receptors in the brain of monkey and man using PET and labeled nicotine as a ligand for the receptors. Progress in Brain Research, Nordberg et al. (Ed.), 79:313-19 (1989).
None of the prior art discloses a successful noninvasive method for imaging nAChRs in the living brain. Most prior methods could not be performed on living brain tissue. Although both nicotine and cytisine have been used in the living brain, they have been unsuccessful due to their non-specific binding affinity and their rapid clearance from the brain.
Despite the various compounds and methods disclosed in the prior art, there remains a need for a noninvasive method of imaging and quantifying nAChRs in the living human brain, to allow for research studies and the diagnosis of neurodegenerative diseases, including, for example, Alzheimer's disease.